SOIL HEAT 



305 



of heat by the soil is through radiation, this radiation 

 being controlled by certain factors of which moisture 

 content, soil mulches, artificial coverings, shelters, and 

 clouds are the most important. Color as a factor in 

 radiation has already been eliminated by the work of 

 Bouyoucos and Lang. The effects of texture and struc- 

 ture have also been investigated by these authors, as 

 well as by other physicists. The general results seem 

 to indicate that unless a dry soil is dealt with these factors 

 may be eliminated from consideration as far as their 

 direct practical effect on radiation is concerned. Of 

 course, indirectly through their influence on such factors 

 as moisture, they are of extreme importance. 



x\n increase in the moisture of a soil has the general 

 effect of heightening the radiation ratio. This, together 

 with the effects of evaporation and of increased specific 

 heat, accounts for the fact that an undrained soil in spring 

 is a cold soil. Bouyoucos ^ found the following relation- 

 ships between moist and dry soils : - 



Effect 



OF MOISTTJEE 



ON Radiation 



Soil 



Peecentage op 

 moistuee 



Radiation of 

 Moist Soil 



Radiation of 

 Dky Soil 



Gravel 



Sand 



Clay ..... 

 Loam ..... 

 Peat 



4.7 



5.3 



17.2 



25.8 

 84.9 



100 

 100 

 100 

 100 

 100 



92.4 

 93.1 

 91.9 

 90.9 

 86.1 



Mulches, either natural or artificial, tend to check the 

 loss of soil heat through their covering effect and their 



1 Bouyoucos, G. J. An Investigation of Soil Temperature. 

 Michigan Agr. Exp. Sta,,5 Tech. Bui. 17, p. 34. 1913- 



